1. Field of the Invention
The embodiments of the present invention relate generally to semiconductor devices and, more particularly, to high-gain bipolar junction transistors.
2. Description of Related Art
Bipolar junction transistors (BJTs) are three-terminal devices having emitter, base, and collector terminals. In typical common-emitter applications, a relatively small base (input) current controls a relatively large collector (output) current. Whether in power or small-signal applications, a large ratio of change in collector current to change in base current is desirable, this ratio often being referred to as hFE or β. A BJT also exhibits breakdown voltages often described as collector-base breakdown voltage, BVCBO and a collector-to-emitter breakdown voltage, BVCEO. These three parameters, β, BVCBO and BVCEO are not independent but rather are interrelated by structural limitations of physical devices. For example, with BVCBO held constant, β varies inversely with BVCEO, while β varies directly with BVCBO when BVCEO is held constant. These interrelationships place an upper limit on attainable values of β when prior-art methods are used to fabricate NPN BJTs having specified breakdown voltage limits.
A need thus exists in the prior art for a method of manufacture that produces an NPN BJT having high values of β even with large values of BVCEO.